Thermal decomposition is a chemical reaction where heat causes one substance to break into two or more different substances. The heat is used to break down the bonds holding the atoms of the original molecules together, so the reaction usually consumes more heat energy than it releases.
The most common materials that undergo thermal decomposition are metal carbonates, along with some metal oxides.
There are cases where thermal decomposition actually releases heat, despite the fact that it requires external input to start the reaction. In such cases, the reaction can begin to fuel itself, sometimes even causing an explosive chain reaction.